BiQuad frequency response for visualisation

Hi there,
I'm trying to make a simple parametrical equalizer using the
Audio-eq-cookbook (like everyone else :-)). I use it on audio signal and
need to visualise resulting frequency response. E.g. peak filter should be
painted as a hat-style curve. The problem is, that when I use the attached
trans****t function H(z) the result is pretty different. It seems, that it
modifies even frequencies it should not and most of all the amplitude
response is not equal for all frequencies (actually there are huge
differencies). But if I use some kind of professional equalizer and e.g.
move the frequency, its graph seems to correctly move left-wards or
right-wards with no amplitude changes - "hat moves to the left or right".
Why is that?

Thanks a lot.